Methods for screening and diagnosing diseased states within the body are based on sensing a physical characteristic or physiological attribute of body tissue, then distinguishing normal from abnormal states from changes in the characteristic or attribute. For example, X-ray techniques measure tissue physical density, ultrasound measures acoustic density, and thermal sensing techniques measure differences in tissue heat. Another measurable property of tissue is its electrical impedance, i.e. the resistance tissue offers to the flow of electrical current through it. Values of electrical impedance of various body tissues are well known through studies on intact humans or from excised tissue made available following therapeutic surgical procedures. In addition, it is well documented that a decrease in electrical impedance occurs in tissue as it undergoes cancerous changes. This finding is consistent over many animal species and tissue types, as summarized by Pethig and Kell.sup.1. Human breast cancers, in particular, have shown similar changes in studies such as those of Chaudhary et al..sup.2 and Surowiec et al..sup.3 Both groups examined surgically excised normal and malignant human breast tissue and obtained similar results, i.e. on average, the electrical impedance of breast cancer tissue was about one-third that of the normal surrounding breast tissue.
Electrical impedance imaging has been proposed to create a picture of electrical impedance differences within a body region.sup.4,5,6 much as an X-ray provides a picture of differences in physical density. One of the incentives to do so is the potential application of electrical impedance imaging as a screening technique for breast cancer, either as a replacement of or supplement to X-ray mammography. Mammography has reasonable sensitivity for detecting abnormalities when present, but the technique fails to detect about 5 to 15% of breast cancers. This is due to several factors, including concealment of the cancer by overlying normal, but dense, breast tissue, failure of mammography to image certain portions of the breast, as well as errors in perception. Mammography has relatively low success for distinguishing malignancies from other abnormalities, and of the approximately 500,000 breast biopsies performed in the United States each year because of an abnormality detected on mammography, only 15 to 30% of the biopsies reveal cancer. This lack of specificity not only results in needless anxiety and an unnecessary procedure, but adds a significant cost to the breast cancer screening program.
There have been a number of reports of attempts to detect breast tumors using electrical impedance imaging..sup.4,5,6,7,8,9,10 However, there are basic problems when trying to construct an image from impedance data. The paths through tissue of X-rays are straight lines. In contrast, electrical current does not proceed in straight lines or in a single plane; it follows the path of least resistance, which is inevitably irregular and three dimensional. As a result, the mathematics for constructing the impedance image is very complex and requires simplifying assumptions that greatly decrease image fidelity and resolution. Not surprisingly, in view of the image reconstruction difficulties, either no clinical data were published in any of these reports, or if they were, the images were of low resolution and difficult to interpret.